Extensive research is being conducted to provide various Quality of Service (QoS) features with a data rate of about 100 Mbps in the advanced fourth-generation (4G) communication system. The 4G communication system is evolving to provide mobility, high data rate transmission, and high QoS in a broadband wireless access (BWA) communication system such as a Local Area Network (LAN) system and a Metropolitan Area Network (MAN) system. A typical example of the above system is identified in the Institute of Electrical and Electronics Engineers (IEEE) 802.16 communication system.
The IEEE 802.16 communication system uses an orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA) scheme in order to support a broadband transmission network in a physical channel of the wireless communication system.
In an OFDM/OFDMA communication system, one OFDM symbol received at a base station is the sum of signals of user terminals (or mobile stations). Thus, there is a timing offset that is a time error between signals for each subcarrier in one OFDM symbol. The timing offset occurs when the timing of each user terminal is inaccurately controlled in a ranging process or when the controlled timing of each user terminal changes slightly due to the movement of the user terminal. Thus, the phase of a signal for each subcarrier changes linearly. Therefore, the timing offset can be estimated by detecting the phase change of pilot signals in the same OFDM symbol.
FIG. 1 is a diagram illustrating the distribution of pilot signals of an uplink single input, single output (SISO) Partial Usage of SubChannels (PUSC) subchannel in the IEEE 802.16 communication system. The structure of a tile is illustrated in FIG. 1. Referring to FIG. 1, four (4) frequency tones and three (3) OFDM symbols constitute one tile including four (4) pilot signals. Thus, in the uplink SISO PUSC subchannel, the timing offset can be estimated and compensated on a tile-by-tile basis by the use of pairs of pilot signals (e.g., P0 and P1) that are paired in each OFDM symbol.
Hereinafter, a description is given of a process for estimating the timing offset by using the pilot signals. First, the relationship between a pair of pilot signals in each OFDM symbol can be expressed as Equation 1:
                                          P            1                    =                                    P              0                        ⁢                          exp              ⁡                              (                                  j                  ⁢                                                                          ⁢                  2                  ⁢                                      π                    ·                    k                    ·                                          t                      s                                        ·                    3                                    ⁢                                                                          ⁢                  Δ                  ⁢                                                                          ⁢                  f                                )                                                    ,                              (                                                            t                  s                                =                                  1                                      F                    s                                                              ,                                                Δ                  ⁢                                                                          ⁢                  f                                =                                                      F                    s                                                        N                    FFT                                                                        )                    .                                    [                  Eqn          .                                          ⁢          1                ]            In Equation 1, k denotes a timing offset, ts denotes a sampling time interval, Fs denotes a sampling period, Δf denotes a subcarrier interval, and NFFT denotes an Fast Fourier Transform (FFT) size (i.e., the total subcarrier number). Herein, noise components are excluded.
For estimation of the timing offset, Equation 1 can be transformed into Equation (2):
                                          P            0            *                    ⁢                      P            1                          =                                                                          P                0                                                    2                    ⁢                                    exp              ⁡                              (                                  j2π                  ·                                                            3                      ⁢                      k                                                              N                      FFT                                                                      )                                      .                                              [                  Eqn          .                                          ⁢          2                ]            In Equation 2, k denotes a timing offset and NFFT denotes an FFT size
Equation 2 can be rearranged for the timing offset k as Equation 3:
                    k        =                              1                          2              ⁢              π                                ·                                    N              FFT                        3                    ·                                                    tan                                  -                  1                                            ⁡                              (                                                      Im                    ⁡                                          (                                                                        P                          0                          *                                                ⁢                                                  P                          1                                                                    )                                                                            Re                    ⁡                                          (                                                                        P                          0                          *                                                ⁢                                                  P                          1                                                                    )                                                                      )                                      .                                              [                  Eqn          .                                          ⁢          3                ]            In Equation 3, NFFT denotes an FFT size, Im( ) denotes an imaginary component, and Re( ) denotes a real component.
One uplink PUSC slot (i.e., the minimum resource allocation unit) includes six (6) tiles. Thus, for reduction of a noise effect, a timing offset can be calculated by accumulating the conjugate products of pilot pairs for each tile in every slot of a burst received from a user terminal, as Equation 4:
                    k        =                              1                          2              ⁢              π                                ·                                    N              FFT                        3                    ·                                                    tan                                  -                  1                                            (                                                Im                  [                                                            ∑                                              N                        slot                                                              ⁢                                                                  ∑                                                  N                          tile                                                                    ⁢                                              (                                                                                                            P                              0                              *                                                        ⁢                                                          P                              1                                                                                +                                                                                    P                              2                              *                                                        ⁢                                                          P                              3                                                                                                      )                                                                              ]                                                  Re                  [                                                            ∑                                              N                        slot                                                              ⁢                                                                  ∑                                                  N                          tile                                                                    ⁢                                              (                                                                                                            P                              0                              *                                                        ⁢                                                          P                              1                                                                                +                                                                                    P                              2                              *                                                        ⁢                                                          P                              3                                                                                                      )                                                                              ]                                            )                        .                                              [                  Eqn          .                                          ⁢          4                ]            In Equation 4, NFFT denotes an FFT size, Im( ) denotes an imaginary component, Re( ) denotes a real component, Nslot denotes the total slot number, and Ntile denotes the total tile number.
However, in the case of multiple input, multiple output (MIMO) environments not the SISO environments, antennas use pilot signals in a distributive manner as illustrated in FIGS. 2A and 2B. That is, if there are two antennas, pilot signals, which are present in one pair on each of the time and frequency axes as illustrated in FIG. 1, are distributed to each antenna as illustrated in FIGS. 2A and 2B. In this case, there are no adjacent pilot signals on the same OFDM symbol in the tile, making it impossible to estimate a timing offset in the above-described manner.